Device for measuring a mass flow

ABSTRACT

The invention relates to a device for measuring a mass flow, comprising a measuring wheel ( 3 ) that is supported by a shaft ( 4 ) and that is driven and impinged axially by the mass flow. Said wheel deflects the flow, giving it both radial and tangential speed components. The shaft bears an actuation spur wheel ( 7 ), which engages with an intermediate spur wheel ( 8 ) that is held in position by a force measuring device ( 14 ) and has a second meshing with a drive spur wheel ( 9 ) that is driven by a drive motor ( 13 ). The diameter of the actuation spur wheel ( 7 ) is more than 0.3 fold, preferably 0.5 to 1 of the diameter of the measuring wheel ( 3 ). This provides a device for measuring a mass flow, which has a high zero constant and thus a high degree of measuring precision, is cost-effective to produce, robust during operation and which has none of the disadvantages of prior art.

[0001] The invention relates to an apparatus for measuring a preferablymultiphase mass flow according to the preamble of claim 1.

[0002] In general, in such apparatus bulk material is fed axially onto ameasuring wheel which has radially running guide vanes. The measuringwheel is driven at constant speed and the driving torque is measured.The deflection of the axially introduced bulk material leads to a changein the torque compared with the no-load torque. Changes in the mass flowof the bulk material likewise influence the torque wherein this changein the torque is a direct measure for the fluctuations of the bulkmaterial flow. Such apparatus has a wide range of application in thepneumatic conveying of bulk material of any kind for such differentareas of application as, for example, the iron and steel industry, thebuilding materials industry or chemical process engineering etc.

[0003] In the known apparatus according to the preamble of the mainclaim, the torque or its variation is measured by means of a gear unit.For example, in U.S. Pat. No. 2,771,773 the torque acting on anintermediate shaft arranged between a drive motor and a measuring wheelis measured. This intermediate shaft is held fixed in its position by aspring which directly compensates the no-load torque. The larger torquewhich arises on application of a mass flow to the measuring wheel iscompensated pneumatically. The pressure of a working gas required forthis purpose and read off on a suitable measuring instrument is ameasure for the transmitted torque. However, the proposed type ofpressure regulation involves a comparatively high expenditure onequipment and thus an increased susceptibility to breakdown. Alsodisadvantageous is the system-dependent long dead time which is requiredfor the pressure build-up to compensate for the torque and makes itdifficult to regulate pulsating flows of bulk material.

[0004] A certain improvement is achieved if, in accordance with EP 0 474121 B1, the intermediate shaft is suspended on a force-measuring devicetangentially moveably to the shaft of the measuring wheel or to the axleof the driving spur wheel of the drive motor. However, the aim of EP 0474 121 B1 is to make the measuring device usable for the measurement ofsmaller mass flows with the disturbing influences of friction in thegears, caused for example by the temperature-dependent changes inviscosity of the gear oil, being largely eliminated. A similarformulation of the problem had already been described in DE 35 07 993C2. Disadvantages of the relatively expensive gearing designs proposedin DE 35 07 993 C2 and EP 0 474 121 B1 are the cost-intensivemanufacture and the susceptibility to breakdown caused by the pluralityof moving parts.

[0005] In the apparatus described in DE 35 07 993 C2 and EP 0 474 121 B1it is desired that the force effect of the intermediate shaft on theforce-measuring device should be as large as possible. For this reason,the shaft carrying the measuring wheel carries a driving spur wheelhaving the smallest possible diameter. In practice, however, thisembodiment has the decisive disadvantage of a low zero-point constancy,i.e., fluctuations of the torque occur even during idling. However, anadequate zero-point constancy is a prerequisite for measuring low massflows in the range of 10 kg/min with a high accuracy. In addition, ahigh zero-point constancy ensures that even small changes in themeasured mass flows can be reliably detected.

[0006] EP 0 857 953 A1 describes a reduction in the measurement error,especially during the measurement of inhomogeneous materials. By meansof an axially centric supply of material into the measuring wheel andmodifying the geometry of the measuring wheel, an attempt is made toavoid the disadvantage of a lateral supply of material and achieve anenhanced measurement accuracy. In practice, such forms of measuringwheel are already known. The geometries described are found to beespecially advantageous for fibrous material. An increased zero-pointconstancy and improvement in the measurement accuracies cannot beachieved for small mass flows in the range of 10 kg/min. The measuringwheel geometry described has the disadvantage of requiring highprecision in the manufacture and requiring protection against wear inthe case of abrasive materials in order to ensure the precision requiredfor the measurement accuracy in long-term operation.

[0007] In all known embodiments the bulk material is deflected such thatthe outlet of the measured bulk material does not lie on theperpendicular axis below the bulk material inlet. This primarilyprevents usage in those cases where a measuring apparatus is to be builtinto existing installations. There the apparatus is typically built inbelow a storage container, where the given guidance of the pipingrequires aligning flanges between the bulk material inlet and outlet,and the maximum overall height of the apparatus is severely limited.Said aspects impede the wide-spread applicability of the principle formany conceivable applications for the measurement of multiphase massflows.

[0008] The object of the present invention is thus to provide anapparatus for measuring a mass flow with high zero-point constancy andthus with high measurement accuracy which can be manufactured atfavourable cost, is robust in operation and avoids the disadvantages ofthe prior art.

[0009] This object is achieved by an apparatus for measuring a mass flowhaving the features of claim 1. Advantageous embodiments of theinvention are described in the dependent claims.

[0010] According to the invention, an apparatus for measuring a massflow is provided with a measuring wheel borne by a shaft, wherein themeasuring wheel is driven and is acted upon axially by the mass flow,deflects this flow and imparts thereto a radial as well as tangentialvelocity component. The braking moment caused by the mass flow therebyserves to quantify the mass flow. The shaft carries a driving spur wheelwhich meshes with an intermediate spur wheel which is held fixed in itsposition by a force measuring device and has a second meshing with adriving spur wheel driven by a drive motor, wherein the diameter of thedriving spur wheel is greater than 0.3 times, preferably 0.5 times toonce the diameter of the measuring wheel.

[0011] It has surprisingly been found that by means of this measure thezero-point constancy is improved substantially and the measurementaccuracy increases accordingly. An expansion of the measurement rangeinto the range of smaller mass flows is thereby made possible. Theinfluence of friction caused by the temperature-dependent changes inviscosity of the gear oil however have far less influence on themeasurement accuracy than assumed so far, at least in the temperaturerange above 0° C. Thus, the known measures for compensating forfrictional influences can be dispensed with in a broad temperaturerange. An apparatus according to the invention thus merely consists of asimply constructed measuring gear unit having a low susceptibility tobreakdown, whereby in addition to higher measurement accuracy, there arecost advantages in manufacture and operation.

[0012] The apparatus according to the invention is advantageouslyprovided for measuring a preferably multiphase bulk material flow. Bulkmaterial flows in the range of 5 t/h to 30 t/h are typically measured.However, the apparatus according to the invention can also be used tomeasure small bulk materials flows.

[0013] When dimensioning the driving spur wheel, it should naturally beborne in mind that a larger spur wheel diameter initially leads to alower measuring force on the intermediate shaft. In an advantageousembodiment of the invention, in the force-measuring device holding theintermediate shaft, the measuring force is guided via a leveragesupported axially parallel to the intermediate shaft with the aim ofincreasing the measuring force at the force transducer. The lowermeasuring force at the intermediate shaft resulting from an enlargeddriving spur wheel compared with the prior art is compensated in thisfashion. Since the bulk material is in any case accelerated radiallyoutwards in the measuring channels and a renewed deflection of the bulkmaterial flow only takes place at the outer edge of the measuring wheel,it is possible to have gear structures where the driving spur wheel hasapproximately the diameter of the measuring wheel without anysignificant additional resistance opposing the bulk material. As a rulehowever, a sufficient zero-point constancy is to be achieved even withsmaller diameters. The force guidance via a leverage also allows the useof force transducers adapted to the intended usage. Commercial forcetransducers with measuring ranges below 50 N are only suitable forlaboratory operation. In an apparatus according to the invention theleverage can be selected such that even for small mass flows it ispossible to use force transducers suitable for industrial use withmeasuring ranges above 50 N so that extremely small mass flows, lessthan or equal to 10 kg/min, can be measured accurately even inindustrial use.

[0014] In a further advantageous embodiment the measuring gear unit isaccommodated in a cross-beam that protrudes into the bulk material flow.It is advantageous for this arrangement if the inlet and outlet pointsof the bulk material lie directly one below the other in alignment. Inaddition, this arrangement has only a very low overall height whichcorresponds, for example, to twice the diameter of the measuring wheel.The apparatus according to the invention can thus easily be built intoan existing installation by simply replacing a short straight pipesection by the apparatus.

[0015] Finally, a further advantageous embodiment of the presentinvention provides that the drive motor is constructed as an electricasynchronous motor. By using an asynchronous motor, a favourably pricedand almost maintenance-free drive solution is made available. In thiscase, the measuring wheel is not driven at constant speed, which is whythe drive shaft of the drive motor and/or the motor itself are coupledto a revolution counter. By comparing or recording the change in speedof the drive motor speed during the measurement, the braking moment andthus the magnitude of the mass flow can be determined.

[0016] In an advantageous further development the shaft of the drivemotor can be arranged offset by 60° to 120°, preferably by 90° to thelongitudinal axis of the housing. By this means, an especiallyspace-saving method of construction can be implemented.

[0017] Further developments and advantages of the invention areexplained by describing the embodiments with reference to the appendeddrawings wherein:

[0018]FIG. 1 is a schematic side view of a section through an apparatusaccording to the invention;

[0019]FIG. 2 is a schematic top view of the gear unit of the apparatusaccording to the invention from FIG. 1;

[0020]FIG. 3 is a schematic top view of the apparatus according to theinvention from FIG. 1.

[0021] The apparatus according to the invention shown as a schematiccutaway side view in FIG. 1 consists of a housing 1 which is configuredin the present case as a welded structure. The housing 1 is constructedas substantially rotationally symmetric and corresponds to two truncatedcones with their bases standing one on top of the other.

[0022] At its upper end the housing 1 has a circular material inletopening 2 bordered by a circular flange 18. At its lower end the casing1 has a circular outlet opening 5 also bordered by a circular flange 19.

[0023] On one side of the housing 1 there is also provided a drivingflange 20 to accommodate a drive motor 13.

[0024] Below the material inlet opening 2 on the inside the housing 1has a measuring wheel 3 provided with radial guide vanes coaxial to thelongitudinal axis of the housing 21. In the present embodiment the vanesare slightly profiled.

[0025] In the plane of the drawing the measuring wheel 3 is arrangedrotatably at the upper end of a vertical shaft 4. The flow of bulkmaterial S flowing in axially through the inlet opening 2 onto themeasuring wheel 3 is deflected by the measuring wheel 3 in the radialdirection and accelerated both in the radial and in the tangentialdirection. The bulk material ejected or falling from the measuring wheel3 is deflected to the wall of the housing 1 and extracted from thehousing 1 through the outlet opening 5.

[0026] The deflection and acceleration of the bulk material flow ontothe measuring wheel 3 causes a braking moment at the shaft 4 which isdirectly proportional to the bulk material flow. The bulk material flowis measured by means of a spur gear unit arranged in a gear housing 6,consisting of a driving spur wheel 7 arranged at the lower end of theshaft 4, a moveable intermediate spur wheel 8 held in its position by aforce-measuring device 14 and a driving spur wheel 9. In the presentembodiment the diameter of the driving spur wheel 7 is 0.5 times thediameter of the measuring wheel 3.

[0027] In the embodiment shown the driving spur wheel 9 is driven viatwo bevel gears 10, 11 by a shaft 12 of a drive motor 13 offset by 90°to the axis of the driving spur wheel 9. By this means the measuringwheel 3 moves at constant speed. The deflection of the driving torque by90° allows an especially space-saving method of construction of theapparatus according to the invention. However, the shaft of the drivingspur wheel 9 can also, for example, be driven directly by the motor 13or can be connected to the motor 13 in a known fashion via a gear unit,a chain, a cog belt etc.

[0028]FIG. 2 shows a schematic top view of the gear unit of theapparatus according to the invention from FIG. 1. The movableintermediate spur wheel 8 is held in its position here via a leveragefrom the force-measuring device 14. The force-measuring device 14 is,for example, a commercial force transducer. The leverage consists of alever arm 16 supported in the bearing 15 axially parallel to theintermediate spur wheel 8. The leverages are determined by the differentdistances between the bearing 15 and the intermediate spur wheel 8 onthe one hand and the bearing 15 and the force-measuring device 14 on theother hand. The lower measuring force at the intermediate spur wheel 8resulting from the enlarged driving spur wheel 7 compared with the priorart is compensated by the choice of suitable leverages. Theforce-measuring device 14 shown in FIGS. 1 and 2 is a commercialbending-rod or torsion weighing cell.

[0029]FIG. 3 shows a top view of the advantageous embodiment fromFIG. 1. Here the gear housing 6 is arranged in the housing 1 such thatonly a small part of the area 17 available for free passage of bulkmaterial after deflection in the measuring wheel 3 is blocked by thehousing 6. Assembly in existing installations is accomplished morefavourably by means of a flange 18 at the inlet opening 2 or by means ofa flange 19 at the outlet opening 5. The flanges 18 and 19 are locatedin alignment and thus facilitate straightforward insertion in anexisting pipe or the like.

[0030]FIG. 3 also shows that the diameter of the driving spur wheel 7can approximately correspond to that of the measuring wheel 3 to achievean especially high zero-point constancy without an additional obstaclefor the bulk material flow being formed thereby. The resulting lowermeasuring force at the intermediate spur wheel 8 could be compensated,for example, by a suitable configuration of lever arm 16 and bearing 15.Tests have shown, however, that a driving spur wheel 7 having 0.5 timesthe diameter of the measuring wheel 3 already yields a satisfactoryzero-point constancy and an apparatus according to the invention withthis size ratio has an extremely compact structure. The overall heighthere is approximately twice the diameter of the measuring wheel. LIST OFREFERENCE SYMBOLS 1 Housing 2 Material inlet opening 3 Measuring wheel 4Shaft 5 Outlet opening 6 Gear housing 7 Driving spur wheel 8Intermediate spur wheel 9 Driving spur wheel 10 Bevel gear 11 Bevel gear12 Shaft 13 Drive motor 14 Force measuring device 15 Bearing 16 Leverarm 17 Free area 18 Flange at inlet opening 19 Flange at outlet opening20 Driving flange 21 Longitudinal axis of housing S Bulk material flow

1. An apparatus for measuring a mass flow with a driven measuring wheel(3) carried by a shaft (4), and impinged upon axially by the mass flow,wherein the shaft (4) also bears a driving spur wheel (7) which mesheswith an intermediate spur wheel (8), wherein the intermediate spur wheel(8) is held in its position by a force-measuring device (14) and mesheswith a operative spur wheel (9) which is driven by a drive motor (13),wherein the diameter of the driving spur wheel (7) is greater than 0.3times the diameter of the measuring wheel (3), characterized in that thediameter of the driving spur wheel (7) is preferably 0.5 times to oncethe diameter of the measuring wheel (3), and that the drive motor (13)is designed as an electric asynchronous motor.
 2. The apparatus formeasuring a mass flow according to claim 1, characterized in that theapparatus is provided for measuring a preferably multiphase flow of bulkmaterial.
 3. The apparatus for measuring a mass flow according to claim1 or 2, characterized in that a gear housing (6) is provided in which aleverage (15, 16) supported axially parallel to the shaft of theintermediate spur wheel (8) is provided for the force-measuring device(14) in order to transmit the measuring force.
 4. The apparatus formeasuring a mass flow according to one of the preceding claims,characterized in that at least the driving spur wheel (7) of the shaft(4) and the intermediate spur wheel (8) are arranged on a cross-beam(16) which protrudes into the mass flow in the gear housing (6).
 5. Theapparatus for measuring a mass flow according to one of the precedingclaims, characterized in that the apparatus is provided for measuringvery small mass flows, less than or equal to 10 kg/min.
 6. The apparatusfor measuring a mass flow according to one of the preceding claims,characterized in that the housing (1) of the apparatus has a low overallheight which is smaller than or equal to twice the diameter of themeasuring wheel (3).
 7. The apparatus for measuring a mass flowaccording to one of the preceding claims, characterized in that theinlet opening (2) for the material and the outlet opening (5) of themass flow in the housing (1) are arranged directly one below the otherin alignment.
 8. The apparatus for measuring a mass flow according toone of the preceding claims, characterized in that the shaft (12) of thedrive motor (13) is arranged offset by 60° to 120°, preferably by 90° tothe longitudinal axis (21) of the housing.
 9. A method for measuring amass flow, particularly a multiphase flow of bulk material, wherein theapparatus used for executing the measuring method comprises a drivenmeasuring wheel carried by a shaft driven by an asynchronous motor andimpinged upon axially by the mass flow, wherein the shaft (4) also bearsa driving spur wheel which meshes with an intermediate spur wheel,wherein the intermediate spur wheel is held in its position by aforce-measuring device and meshes with a operative spur wheel which isdriven by an asynchronous motor, wherein the drive shaft and/or themotor are coupled with a speed counter and wherein the measuring methodcomprises the following steps: Detecting the force produced on the forcemeasuring device; Detecting the deviation of speed of the drive speed bymeans of the speed counter; Detecting the breaking moment from the forcemeasured at the force measuring device; Calculating the mass flow fromthe breaking moment and the speed.